The present invention relates to protecting cylindrical elements in offshore operations. More particularly, the present invention relates to protecting cylindrical elements such as drilling and production risers which are under the influence of ocean currents and are potentially subject to problems from drag and from vortex induced vibration ("VIV").
Drilling risers are formed from large diameter tubular goods and serve to enclose the drill string from drilling facilities above the water surface provided on a platform or drilling vessel to the well at the ocean floor. This can be a half mile or more in deepwater developments and the drilling riser is not tied to supporting framework such as the conductor guides in traditional bottom-founded platforms. Floatation modules such as buoyancy cans or syntactic foam modules may be deployed along the length of the drilling riser to render it neutrally buoyant, but horizontal or lateral loading from currents on this long, unsupported run is not alleviated by the addition of floatation modules. Rather, the presence of floatation modules around the circumference of the drilling riser materially increases the profile presented to the current and leads to greater drag and VIV effects.
Unabated, the VIV can lead to premature failure of equipment in high current environments and may require that drilling operations cease in response to temporary high current events such as loop currents experienced in the Gulf of Mexico. Further, lateral load from drag may deform the drilling riser to a bowed shape that presents excessive angles with respect to the derrick at the top and the well at the bottom. As a result, the drill string rotating within the drilling riser contacts the riser wall in passing these transitions and the drilling riser is subjected to excessive wear.
Fairings and helical strakes have been used for drag reduction and/or VIV suppression in drilling risers. However, the fairings and helical strakes have been difficult to install and to handle. Further, normal drilling operations require that the drilling riser be pulled periodically. This requires that the drilling riser be retrieved, section by section, and that the riser sections, floatation modules, and VIV protection system be stowed until run again, section by section. Thus there is a significant need for an improved drilling riser VIV protection system and handling method that facilitates deployment, removal, storage, and redeployment.